How would you go about creating a game patch after you have updated your
game?

There are plenty of commercial apps out there that will compare old
files with new and store just the differences. This process isn't
terribly complicated, but it can get quite tricky (determining offsets
and shifts in the data-set.) They also come with installation tools that
will automatically apply the patches, verify proper versions (so as to
apply the proper patch), etc.

Response provided by Paul Nettle

This article was originally an entry in flipCode's Fountain of Knowledge, an open Question and Answer column that no longer exists.